As with many things associated with airline travel, asking to go on an earlier or later flight on the same day as your original flight used to be free, especially if you were willing to stand by for an empty seat. That's no longer the case. Most recently, United Airlines announced changes to its same day flight change policy effective for travel on or after April 20, 2010 for tickets purchased on or after April 10, 2010. And in February, American Airlines eliminated the free same day standby option, and now requires all passengers to pay for a confirmed same day flight change.

But each airline has a slightly different policy, and the rules, which seem to change at a whim, can be confusing.

If you're traveling on a fully refundable fare, or have elite status in the airline's frequent flyer program, you're all set: no fee to get a confirmed same day schedule change.

Some airlines allow same day changes by phone, others only at the airport; and some, including Airtran and JetBlue, still allow free same day standby, but keep in mind that this is not a confirmed change: you take a chance that there will be seat available. And some airlines give you a full 24 hours to make a same day change, while others allow as few as three hours.

And then there's Southwest, which follows a completely different model: there's no fee at all to change your flight, but if you do so, you'll be liable to pay for any fare difference. If you've bought an advance purchase $49 fare and the last minute "walk up" fare is now $249, you'll pay $200 to make the change.

That said, airlines are notorious for breaking their own rules. If you happen upon a sympathetic check in agent you may end up paying nothing, or if your original flight is overbooked, delayed or cancelled, it will be in the airline's self interest to put you on an earlier or later flight for free.

To see what changes are in the air for same day flight changes, see our updated chart.